1. Field of the Invention
This invention relates to a scanning sonar designed to automatically track underwater targets, such as fish.
2. Description of the Prior Art
A scanning sonar is an underwater sounding apparatus which typically emits an ultrasonic sounding beam in all directions underwater with a specific tilt angle from water surface, receives echo signals returning from underwater targets all around and displays target images based on the echo signals. FIG. 12 is a diagram showing a general principle of the scanning sonar. Referring to FIG. 12, designated by the numeral 50 is a scanning sonar installed on a ship 51, designated by the numeral 52 is a transducer of the scanning sonar 50, designated by the numeral 53 is an ultrasonic transmitting beam emitted from the transducer 52, and designated by the numeral 54 is water surface. The ultrasonic transmitting beam 53 is transmitted underwater simultaneously in all directions around the transducer 52, forming an umbrellalike beam pattern directed obliquely downward with a specific tilt angle δ. After the ultrasonic transmitting beam 53 is transmitted, the transducer 52 produces a receiving beam 57 which is rotated at a high speed in a spiral pattern around the transducer 52 to receive echo signals from fish schools, bottom and other underwater objects. Echoes of such underwater objects are displayed in colors on-screen according to intensities of the received echo signals.
In actual fishing operation, it is important to know not only the locations (distance, direction and depth) of fish schools but also their moving directions and speeds in order that fishermen can correctly deploy fishing gear. For this reason, some types of scanning sonars offer a target tracking feature which enables users to automatically track their aimed fish school. FIGS. 13A-13C are diagrams showing examples of pictures shown on a screen 60 presented by this kind of automatically tracking scanning sonar, in which the numeral 61 indicates an own ship mark shown on the screen 60, the numeral 62 indicates a blip representing an echo of a fish school, and the numeral 63 indicates a target lock mark placed on the echo 62 of the fish school to automatically track it. The target lock mark 63 can be entered at a desired position on the screen 60 by performing a particular operation on an operator panel which is not shown.
To start automatic tracking, an operator places the target lock mark 63 on the echo 62 of the fish school to be tracked as shown in FIG. 13A. After commencement of automatic tracking, the scanning sonar 50 adjusts the tilt angle δ of the ultrasonic transmitting beam 53 based on a motion vector of the own ship mark 61 and the location of the target lock mark 63 such that the center of the ultrasonic transmitting beam 53 emitted from the transducer 52 hits the exact position of the target lock mark 63. Consequently, the transducer 52 can receive the echo 62 reflected from the target fish school and the scanning sonar 50 can determine an updated location of the target fish school from echo signals returning from around the position of the target lock mark 63 and updates the on-screen position of the target lock mark 63 accordingly. The scanning sonar 50 performs its automatic tracking function by repeating such operation, whereby a past track 64 of the fish school is drawn on the screen 60 with the lapse of time as shown in FIGS. 13B and 13C.
FIG. 14 is a diagram showing how the scanning sonar 50 detects a fish school 55 with the ultrasonic transmitting beam 53 directed obliquely downward at a tilt angle δ1, in which the fish school 55 is located on the surface of the umbrellalike beam pattern above sea bottom 56. When the fish school 55 moves as shown in FIG. 15, the scanning sonar 50 tracks the fish school 55 by automatically adjusting the tilt angle δ from δ1 to δ2 so that the ultrasonic transmitting beam 53 is continuously directed toward the fish school 55.
The conventional scanning sonar having the aforementioned automatic tracking feature scans only horizontally by rotating the receiving beam 57 along the conical beam pattern. Therefore, if the aimed fish school 55 moves away from the beam pattern area at relatively a high speed, the scanning sonar will fail to continue tracking of the fish school 55. For the sake of simplicity, let us assume that the horizontal distance to and the direction of the fish school 55 remain unchanged and the fish school 55 moves only vertically. As long as the fish school 55 remains within the beamwidth of the ultrasonic transmitting beam 53, the tilt angle δ1 is not varied and the on-screen position of the tracked fish school 55 remains the same even when the fish school 55 moves vertically from the position shown in FIG. 14. If, however, the fish school 55 moves away from the ultrasonic transmitting beam 53 as shown in FIG. 16, the conventional scanning sonar would not be able to adjust the tilt angle δ of the ultrasonic transmitting beam 53 to follow the moving fish school 55 by just scanning horizontally, because the depth of the fish school 55 is unknown. As a consequence, the scanning sonar could no longer continue automatic tracking of the fish school 55 and present its blip on-screen.